Coenct Sharepoint Intranet with external PHP site

We have a Sharepoint Intranet and have to make a link to a php site. Each user logs into Windows with his username and password and then access to the SharePoint intranet. In this Intranet, there is a page that has a link to an external PHP site. So we need to show and send the sharepoint username in the link embed, like this: Or in any other way that solves this. In the external PHP site, we have to GET the username, to identify the user. I hope someone could help us with this, it appears to be not difficult.